Localised insecticide treatment for the control of vine weevil larvae ( Otiorhynchus sulcatus) on field-grown strawberry

Abstract

Three field experiments at Horticulture Research International (HRI), Efford, in 1992, 1994 and 1995, showed that the incorporation of controlled-release chlorpyrifos granules (78–208 g a.i. m −3) into the compost of the propagation modules of strawberry plants gave significant control of vine weevil larvae in the field. Better control (approximately 90%) was achieved when field planting was in early August so that the root system was small at the time of egg infestation, than following planting in mid-May (approximately 50% control) where a much larger root system had grown. Control was as good where the treated modules had a compost volume of 80 ml as where the volume was 230 ml. Better control occurred when eggs used for artificial infestation were placed close to the crown of the plant than when placed 15 cm away. In a replicated field experiment at Hinton Admiral, Hampshire, in 1994, pre-planting spot, band or whole-bed soil treatment of raised-bed, polythenemulched plants (planted as bare-root runners) with the chlorpyrifos granules (at 52 or 104 g a.i. m −3) did not reduce the numbers of larvae significantly. In a further field experiment at HRI East Mailing in 1994, treatment of a 15 cm diameter by 15 cm deep cylinder of soil round each plant (at 104 g a.i. m −3) did not significantly affect larval numbers. Pre-planting spot treatment with imidacloprid granules (125 g a.i. ha −1) or a curative drench (at the same dose) was not efficacious, though good control was achieved with a standard curative drench of chlorpyrifos (13.1 kg a.i. ha −1). The survival of vine weevil eggs and of young larvae was low (less than 9%), circumstantial evidence pointing to soil type and condition as being important determining factors. Lighter soils with a structure allowing easy movement of larvae appeared to be more favourable for the survival of the pest. Where adults were caged round strawberry plants with the surrounding surface soil replaced by sand in the field, most eggs (more than 79%) occurred in the top 0–1 cm of sand, 50% being found on or close to the surface (0 – 0.2 cm depth) in one experiment. Eggs were aggregated weakly round a single non-mulched plant, but there was little evidence of such aggregation round plants grown in polythene-mulched, raised beds. Survival to the semi-mature larval stage from eggs placed on, or 2 cm below, the soil surface 15 cm from the crown of the plant was as great as for eggs placed on, or 2 cm below, the surface adjacent to the crown. Larvae were shown to migrate towards the crown of the plant during their development. Implications for the optimum placement of insecticide granules are discussed.